The face of the earth, ever since its creation some 4.6 billion years ago, has been in a state of constant evolution. The shape and position of the continents (emerged areas) and oceans (submerged areas) have ever been slowly but constantly changing through the geological times. These are because the earth releases large amount of its internal, radioactively derived energy to the surface through the convection currents circulating within the mantle and causing perpetual displacement of the lithospheric plates relative to one another. The palaeogeographic evidences reveal the existence of three supercontinents at different geological times. The Ur and Rodinia supercontinents are known to have existed some 1.5 billion and 750 million years ago. The last of the supercontinents was the Pangaea, surrounded by a mega-ocean named Panthalassa, whose eastern arm was called Tethys. At the beginning of the Mesozoic era some 250 Ma ago during Norian age, the Pangaea was still intact as a single unit.
Configuration of Continents: Norian Age, CGMW, 2003
At around 180 Ma during Toarcian age, the first break-up of the supercontinent started, giving rise to the continents of Laurasia and Gondwana. In the Kimmeridgian age (145 Ma), the Gondwana separated into two parts; the western one included the continents of South America and Africa, and the eastern - of India, Australia and Antarctica. In the Cenomanian age, i.e. the middle of the Cretaceous period (95 Ma), the components of the East and West Gondwanaland split-up along distinct plate boundaries, an N-S trending Atlantic Ocean opened up and India separated out from Madagascar. In the Maastrichtian age, the Atlantic Ocean opened up further, Australia drifted away from Antarctica and India advanced towards the Northern Hemisphere.
Configuration of Continents: Maastrichtian Age, CGMW, 2003
At the Cretaceous-Tertiary interface, also referred to as K-T boundary (65 Ma), major changes took place on the face of the earth. The impact of a 10 km diameter meteorite at Chicxulub in Mexico contributed to a major environmental change that brought about the extinction of 75% of the living species, including the Dinosaurs and the Ammonites. Immense volcanic flow - the Deccan Traps - covered one-third part of India as it passed over a hot spot at Reunion Island owing to accretion of more than 16 cm/year at the Indian Ocean ridge. During the Lutetian age-Palaeogene period (45 Ma), the continents started acquiring their present day position. The period was marked by major orogenies. In North America, Rocky Mountains were formed, India started colliding with Eurasia, thereby initiating the building of the Himalaya and the Pyrenees were born. In the Tortonian age, i.e. Late Neogene (10 Ma), the configuration of the continents and oceans was similar to that of the present day.
Configuration of Continents: Tortonian Age, CGMW, 2003
Towards the last phase of the Quaternary in Pleistocene time, the last Glacial Maximum occurred at 18,000 years B.P. At this time the humans were already present on the earth for some 2.5 million years. The ice sheets covered an area of 25 million sq km of the Northern Hemisphare and the average global temperature was 4.5°C lower than that of today. On a worldwide scale the sea level was 125 m lower, exposing some 20 million sq km of the continental shelf.
Plate Tectonics
It can be said that the foundation of the concept of Plate Tectonics was laid in early 20th Century when a German Meteorologist Alfred Wegener presented the basic tenets of continental drift in 1912 and introduced the name Pangaea to the supercontinent, which existed at the beginning of Mesozoic era. In 1928, a British Geologist Arthur Holmes propounded the hypothesis that the molten material below the lithosphere circulated or convected to provide the driving force for the continents to drift apart. In late 1930’s, Hess, a Geologist at Princeton University during his cruises mapped the seafloor topography and identified the presence of nearly linear ridges-the so-called mid-oceanic ridges-away from which ocean depth increased symmetrically. In 1962, Hess published a landmark paper titled, “History of Oceanic basins”, which presented a mechanism for seafloor spreading. In 1963, a Canadian Geophysicist J. Tuzo William suggested that the characteristic arc shape of Hawaii Islands resulted from the passage of the crust over an upwelling of magma, referred to as the hot-spot, that remained as stationary in the mantle. This concept became critical to the development of the theory of plate tectonics. Other scientists who made significant contributions in the development of the concept include Morley, Vine, Mathews, Allan Cox, Richard Doell, Brent Dalrymple, Teddy Bullard, Dan Mackenzie, Bob Parker and Jason Morgan.
However, till the early part of 1960, there were only a few takers of the theory of plate tectonics. In early 1960’s, the Worldwide Standard Seismograph Network (WWSSN) was launched with the basic objective to monitor the underground nuclear explosions. The seismic data obtained from the network helped to a large extent in building the structure of the earth. The concentration of the seismic events along certain linear belts around the globe revealed in no uncertain terms the existence of mega fractures or tectonic discontinuities that segmented the crust into discrete blocks or plates. It was established that the continental plates, constituting the lithosphere are on an average 70 km thick, underneath which lays the partially molten aesthenosphere. The slow convection of the aesthenosphere is the engine that drives plate tectonics; earthquakes are in a sense the transmission.
The continental plates diverge away from each other at the mid-oceanic ridges and subduct underneath another at the plate margins. If the continental plates cannot subduct any more, the process of continental collision begins, wherein the uplift of the earth mass and mountain building activity takes place. The Himalaya, whose creation began some 40 Ma ago and still continuing, is a product of such a collision.
A small part of the subducted oceanic crust, warmed up during its slow descent into the asthenosphere, dehydrates and locally may even begin to melt at about 100 km depth. In this process it releases fluids that will subsequently cause partial melting of the overlying mantle, which forms an asthenospheric wedge between the subducted crust and the overthruting lithosphere. This upwelling of magma is typical of subduction and it creates a string of volcanoes along the overthrusting plate margin. The Andaman-Sumatra Volcanic Arc, where the only active Indian Volcano is situated at the Barren Island, is an example of such a process. The eruptions along the overthrusting plate margins are explosive type and those centered along the mid-oceanic ridges, effusive type.
Barren Island volcano eruption - January 2009
Earthquake
An earthquake is a series of vibrations on the surface of the earth caused by the generation of elastic waves due to sudden rupture of fault segments within the earth. In fact, it is just a manifestation of the processes of tectonic adjustments and disturbances occurring ceaselessly within the earth, and could number a few hundred thousand every day. Fortunately, most of the seismic releases are of such small order that only sensitive instruments can sense them or they get centered at locations far away from human settlements. More than 90% of the earthquakes occur in regions where large tectonic plates have their boundaries, which could be either convergent (subduction or continental collision zones), divergent (mid-oceanic ridges or continental rifts) or transform. In the terminology of seismology, this is referred to as the ‘interplate’ zone. The 40,000 km long Circum-Pacific seismic belt, demarcating the boundary of the Pacific Plate, accounts for almost 80% of the Global seismicity. The second most significant Plate boundary is the Alpine-Himalayan seismic belt. All tectonic plates have internal stress fields as well, caused by their interactions with neighbouring plates and loading/unloading of large superincumbent mass (e.g. deglaciation). Relatively longer recurrence period earthquakes occur in such tectonic provinces, referred to as ‘intraplate’ zone.
Earthquakes are commonly held to be the most destructive of all natural forces. For reasons of their unpredictive and erratic behaviour, suddenness of occurrence and impact over very large areas, these are the most feared of all the hazards. An analysis of the Worldwide Earthquake database for the period 856-2009 reveals that in the last 1153 years, 879 earthquakes of historic importance have occurred round the globe in which more than 4.085 million human fatalities have been recorded. Of this, 6 events were of magnitude (M) = 9.0 (Table-1), 78 of M = 8 = 9 and 301 of M = 7 = 8. In 20 events, the fatalities were = 50,000 (Table-2), in 28, = 10,000 = 50,000 and in 100, = 1,000 = 10,000.
Table-1 Earthquakes of Magnitude = 9.0 of the World
Most Fatal Earthquakes of the World
The Indian subcontinent, since time immemorial, has witnessed numerous destructive earthquakes, particularly along the plate margins. It is estimated that in a period spanning for 186 years from 1819 to 2005, more than 189,000 persons have perished due to the primary and secondary effects of the seismic events. The 2,400 km long Himalayan arc, where continental collision is active, accounts for 5% of the global seismicity. Here, in a relatively short period of 53 years between 1897 and 1950, four great earthquakes inflicted heavy damage to life and property. The N-S trending Tertiary mobile belt, extending from Nagaland in the north to the Andaman-Nicobar group of Islands in the south, forms the active plate margin and is marked by profuse seismic outbursts. In the Andaman region the oceanic crust of the Indian plate subducts below the Burmese plate. The peninsular India, constituting the intraplate region, can broadly be divided into two tectonic domains of i) craton or the stable continental shields of Aravalli, Bundelkhand, Dharwar, Bastar, Singhbhum, etc. and ii) the fossil rift belts of Kachchh, Cambay, Son-Narmada-Tapti (SONATA), Mahanadi, Godavari, etc. Moderate seismic activity has been the characteristic of Peninsular India with the exception of Kachchh graben, where earthquakes in excess of M7.0 have occurred on more than one occasion and the seismic energy release has been at the average rate of 9.9 x 1020 ergs/year, which is quite comparable with that of the plate margins.
Major Earthquakes of the Indian Subcontinent
The Indian landmass, susceptible to different levels of seismic hazard, has been classified into four distinct Seismic Zones as per the Seismic Zoning Map of India contained in IS 1893: 2002-Fifth Revision. Zone V, covering 10.9% of the country’s area, is liable to Seismic Intensity MSK IX and above (general destruction) and is referred to as Very High Damage Risk Zone in the Vulnerability Atlas of India-First Revision. It includes parts of Jammu & Kashmir, Himachal Pradesh, Uttarakhand, North Bihar, entire North Eastern Region, Andaman & Nicobar group of Islands and major part of Kachchh district. Zone IV, constituting 17.3% of the land area, is liable to MSK-VIII (destruction of buildings), and has been termed as High Damage Risk Zone. It encompasses the remaining part of the Himalaya, part of Gujarat, Koyna region and northern part of Indo-Gangetic plains, including Delhi. Zone III, occupying 30.4% of the country’s area, is associated with Intensity VII (damage to buildings) and comprises Moderate Damage Risk Zones. It broadly encompasses the rift systems of Peninsular India, West Coast, southern part of the Indo-Gangetic plain as well as the Lakshadweep group of Islands. Zone II, now merged with Zone I of the earlier maps, includes 41.4% of the landmass, where the probable Intensity could be VI or less (frightening). Termed as Low Damage Risk Zone, it is broadly associated with the stable continental shields of Peninsular India.
Map showing Seismic Zones of India, IS 1893 (Part-1), 2002
The Kachchh Earthquake of 16 June 1819 had a profound effect in the entire peninsula and brought in some spectacular terrain deformation in the Great Rann. Burnes (1835), in his account of the catastrophe writes, “By the severe shock, hundreds of people perished and every fortified stronghold was shaken to its foundation. Innumerable wells and rivulets were changed from fresh water to salt water. The Brick Fort of Sindhri was overwhelmed at once with a tremendous inundation of water from the Ocean, converting the area into a 25 km long lake”. A remnant of Sindhri lake still exists.
Isoseismal map of 8 October 2005 Kashmir earthquake (GSI, 2006)
Damage of a masonry structure at Salamabad by 2005 Kashmir earthquake
Damage to a structure in the epicenter of 2001 Bhuj earthquake
Liquefaction crater, 2001 Bhuj earthquake
Ground deformation, 2001 Bhuj earthquake
Coseismic reactivation of a fault displacing pebble bed, 2001 Bhuj earthquake
Tsunami
Tsunami is a Japanese word meaning harbour waves (tsu: harbour, nami: waves), which can be caused by an off shore earthquake, a seabed volcanic eruption or a marine landslide. The off shore earthquakes, the most frequent cause of tsunami, are located along the subduction zones where one lithospheric plate is in the process of sliding beneath the other overriding plate. Earthquakes related to the subduction zone are generally of four types; i) shallow interplate thrust events, ii) shallow earthquakes caused by deformation within the upper plate, iii) earthquakes at depths from 40 to 700 km and iv) earthquakes seaward of the trench. A major earthquake, usually of magnitude 7 or more may rupture a large segment of the subducting plane. If the mechanism is thrust type then the sea floor could suddenly be thrown up, producing a pedaling effect, which could give rise to large sea waves or tsunami. A strike-slip rupture, where there is insignificant vertical displacement of the sea floor, will fail to generate a tsunami, howsoever big the magnitude may be. In case of the 26 December 2004 earthquake of magnitude 9.3 and focal depth of 30 km, a 1,200 km long segment of the Andaman-Sumatra trench ruptured and the sea floor thrown up by as much as 5-6 m to generate the giant tsunami in the Indian Ocean.
A tsunami is different from the normal sea waves in the sense that it has large wavelength, greater amplitude, larger period and much greater velocity. A typical tsunami wave has 10-12 m height, 200 km wavelength and a speed of 700-900 km/hr in deep waters. They contain about 1% to 10% of the total energy of the earthquake, which in the event of Great earthquakes is of the order of 1023 or 24 ergs. Since the velocity of tsunami is a function of depth, which is expressed as c = vgD, where c is velocity, g acceleration and D water depth, the wave speed decreases on approaching the continental coasts. As the period remains constant and the velocity decreases, the wavelength gets shortened and wave amplitude increases from less than a metre in deep sea to as much as 30 m in the shores. The tsunami waves reach the shores in successions of troughs and crests. If the trough portion of the wave reaches the shore first, it draws down the water, exposing the seabed even to distances of kilometers. This phenomenon was witnessed in the 26 December 2004 tsunami at some places. People in the Andaman coasts were amazed to see the sea front receding and exposing the seabed and coral reefs.
The effect of the tsunami can be different at different places, depending upon the topography of the coast. The destruction is principally caused by the lateral impact as well as inundation. The retreating waves have also considerable velocities and tend to carry loose objects, people and livestock. The extent of damage and loss of life are related to run-up height, velocity, local topography, population density and land use pattern, apart from the timing of the event and availability of alert systems. Erosion of the land surface and deposition at places, besides flooding of the low-lying areas and fresh-saline water mixing are usually associated with a tsunami.
The Pacific Ocean is the home of most of the tsunami. According to the Tsunami Laboratory in Novosibirsk, a total of 796 tsunamis were recorded during the period from 1900 to 2001, of which 117 caused damage and casualties. A maximum number of 19 tsunamis occurred during 1938 though without causing any damage. In the Pacific region 17% of the tsunamis were generated in or near Japan. South America accounts for 15%, New Guinea Solomon Islands 13%, Indonesia 11%, Kuril Island and Kamchatka 10%, Mexico and Central America 10%, Philippines 9%, New Zealand and Tonga 7% and Hawaii 3% of the total tsunamis. A maximum run-up of 30 m was recorded at Hokkaido in the 7th December 1933 tsunami at Awa, Japan that killed 200 persons. The 1703 tsunami at Awa, Japan killed more than 100,000 people. Underwater volcanic explosions of Krakatua Island on 26-27 August 1883 caused waves as high as 35 m in many East Indies localities that resulted in the loss of more than 36,000 human lives.
Records of some 16 tsunamis are available from the Indian Ocean since 1762. However, the 2004 event was the most destructive. Countries severely affected in the Indian Ocean by this calamity include Indonesia (more than 200,000 fatalities), Sri Lanka (30,000 fatalities), India (about 10,000 fatalities), Thailand, Malaysia, Maldives, Myanmar, Bangladesh, Somalia, Tanzania, Seychelles and Kenya. This tsunami also had its effect felt in Madagascar, Mauritius, Mozambique, South Africa, Australia, Antarctica, New Zealand and coasts of South and North America. The Geological Survey of India carried out multidisciplinary study of the earthquake-tsunami the outcome of which has been produced in GSI Special Publication No. 89, 2007.
Physiographic map of part of Indian Ocean-Subcontinent (modified after CGMW Sheet-1, 2002)
On 26th June 1941, an earthquake of magnitude 8.1 occurring in the Andaman coasts also generated tsunami though figures of any damage are not available. On 27th November 1945, a M8.0 earthquake, occurring along the Makran subduction zone off the Pakistan coast, caused tsunami where 4,000 fatalities took place. The coastal tracts of Gujarat and Maharashtra were also affected by the tsunami.
Data Source: i) The Changing Face of the Earth, Commission of the Geological Map of the World (CGMW), 2003.ii) Hough, S.E., 2002. Earth Shaking Science. Princeton Univ. Press.iii) IS 1893 (Part 1), 2002. Criteria for earthquake resistant design of structures.iv) Vulnerability Atlas of India-First Revision, 2006, BMTPC Publication.v) GSI Earthquake Publicationsvi) Pande, P., 2008. Evaluation of seismotectonics of Kachchh rift basin…PhD Thesis.
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